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High-Resolution Nanochemical Mapping

Figure 3 : Analysis of a two-phase polymer. (a) S-SNOM refl ection and (b) absorption signals at the third harmonic of a PS-PMMA blend with topography in (c). (a) and (b) show a constant refl ection and vanishing absorption of the PS matrix for different infrared frequencies where the PS material with its fl at, absorption-free dispersion serves as a reference material. On the contrary, the PMMA domains exhibit a distinct wavelength-dependent behavior, as extracted for the large domain in panel (d) for multiple frequencies. Both refl ection and absorption can be fi tted with a common Lorentz oscillator model (solid lines).

of the cantilever resonance frequency, with good background suppression where the tapping frequency Ω is tens to hundreds of kilohertz at a typical probe oscillation amplitude of several tens of nanometers. Signal analysis . To enhance the weakly scattered near-fi eld signal of the s-SNOM and to obtain the electric fi eld and the phase of the scattered light, an asymmetric Michelson- type interferometer is employed. This technique represents a straightforward way to obtain the near-field absorption and refl ection signals, the nanoscale analogs of conventional far-field FTIR absorption, and reflection signals. A typical configuration for such a homodyne detection scheme is depicted in Figure 2a . T e light of the infrared light source is split with a 50:50 beamsplitter to provide a reference beam that is refl ected from a piezo-controlled mirror and then focused on a mercury cadmium telluride (MCT) detector together with the scattered light from the AFM tip. T e recorded interference signal on the detector V contains the near-fi eld amplitude and phase, E nf and φ nf respectively, and the corresponding values for the reference ( E r , φ r ) and the background signals ( E b, , φ b ) in the following way:

+ Eb Enf cos(ϕb – ϕnf) + Er Eb cos(ϕr – ϕb) + |Enf|2 + |Er|2 + |Eb|2

V α Er Enf cos(ϕr – ϕnf) This equation can be simplified to extract the desired

near-fi eld amplitude E nf and phase φ nf T e last four terms are suppressed by signal demodulation or are negligible compared to the other terms, and the second term can be neglected because E r >> E b . We note that the weak near-fi eld signal is enhanced in the fi rst term by multiplication with the much larger reference

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fi eld. To extract near-fi eld amplitude and phase, a two-phase homodyne detection scheme is employed, that is, the signal is measured in two positions of the reference mirror corresponding to two distinct reference phases φ r . These mirror positions are adjusted with the SNOM tip over a non-absorbing, reflecting material in the infrared region of interest such as Si. In practice, the mirror is translated until a maximum of the demodulated detector signal is found, corresponding to a vanishing phase difference between reference and near-field phase on the Si material. Moving the mirror by 1/8 of a wavelength shifts the reference phase by π /2 and thus suppresses the detector signal entirely on the non-absorbing Si. Aſt er having defi ned the reference mirror positions in the described way, the SNOM tip is placed over the sample of interest. T e detector now measures a signal Vrefl α Er Enf ′ cos(ϕnf ′) (1 st mirror position) and Vabs α Er Enf′ sin(ϕnf′) (2 nd mirror position), corresponding to signals proportional to the refl ection

and absorption of the sample, respectively [ 19 – 21 ]. Note that these quantities represent the real and imaginary part of the complex near fi eld Enf ′ exp(iϕnf ′) from which the amplitude and phase can easily be obtained.

We emphasize that for weak oscillators such as polymers or biomaterials the obtained absorption spectra allow material identifi cation without the need for elaborate modeling. Diff erent models exist ranging from those that allow a qualitative understanding to those that give a detailed, quantitative description of the tip-sample interaction [ 2 , 22 – 24 ]. However, the exceptional agreement of near-fi eld absorption data and standard far-fi eld FTIR data in the case of polymers or biomole- cules has been shown recently in several publications [ 14 , 19 – 21 ] and renders s-SNOM an excellent and modeling-free technique for most practical cases.

PeakForce Tapping mode . An AFM is the basis for s-SNOM operation, and hence s-SNOM can be combined with other AFM modes. PeakForce Tapping is such a mode, and it is particularly suitable for imaging of soſt samples, such as polymers. In PeakForce Tapping, the tip-sample distance is modulated sinusoidally. T e tip-sample interaction in contact is controlled to keep the maximum force or “peak force” constant. T is method of precise control of the tip-sample interaction force captures and analyzes approach and withdraw curves at each pixel with 1–2 kHz repetition rate and at user-defi ned forces down to tens of pN [ 16 ]. One aspect of PeakForce Tapping is that its force feedback preserves the AFM tip and sample and thus enables imaging of delicate samples with ultra-high resolution. T is allows, for instance, routine observation of the major and minor grooves in the DNA double helix [ 15 , 25 ].


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